In much of the prior art it has been an object to try to recover the aluminum content of skim by chemical means as the skim rests on the surface of the molten bath. Recovery was believed achieved by fluxing the bath with, say, chlorine gas or other reactive gasses (which may be used also for conditioning the melt) or by distributing over the skim an "exothermic" mixture of fluxes (salts). The result was a skim or dross layer above the melt which was "dry", i.e. it did not contain as much liquid metal as before treatment. The easy and pleasant assumption was that the metal content had been released and had joined the bath below. That assumption was difficult to refute on commercial furnaces because precise accurate material balances were difficult if not impossible to achieve at that scale. However, small scale experiments such as those reported in part by W. R. Ford and N. Christ in "A Study of the Formation and Behavior of Aluminum Skim". The Metallurgical Society of AIME, Technical Paper No. A74-61 have shown conclusively that, with such treatments, much of the metallic portion of the skim either reacted with the chlorine to form gaseous aluminum trichloride, an atmospheric pollutant or became hot enough to react with the atmospheric gases above the skim to form nonmetallic compounds, quantities of which were "fumed" as fines up the furnace stacks. Thus the metal content of the skim was not reclaimed but was converted to base compounds for an economic and energy loss.
Other prior art has shown the efficacy of removing skim from a furnace and either while it is hot or after it has been cooled treating the skim with molten salts of the chloride type with small proportions of fluorides usually added. These methods have a major disadvantage in that the molten salts typically emit pollutant fumes during processing and that the processing residue, which contains metal, metallic oxides and carbides as well as the residual salt, must be further reclaimed or disposed of in a safe and proper manner. The complexity of such processing is readily seen in such prior art as U.S. Pat. Nos. 4,434,142 and 4,523,949. In earlier times such residues could be discarded in dumps since it was not recognized that said salt-contaminated residues were pollutants.
In other prior art, to avoid the problems associated with salt, skim treatments under inert gases have been used as is typified by the work of Dominic Montagna of the United States Bureau of Mines and covered in U.S. Pat. No. 3,999,980. Such methods have the inherent economic disadvantage of requiring the use of inert gas as well as the additional need for protective atmosphere assurance and control.
There are other prior art methods which seek to recover metal directly from the hot skim by mechanical treatment such as by rolling as exemplified by U.S. Pat. No. 4,772,320; by pressing e.g. U.S. Pat. Nos. 4,386,956 and 4,540,163 or by centrifuging U.S. Pat. No. 4,137,073. These mechanical separation and recovery methods are inherently superior to those employing chemicals such as salts or chlorine gas because, inter alia, non-productive, pollution-abatement equipment is not required and are inherently superior to methods which require inert gas covers because the expense and control of the protective atmosphere is not required.
Of the mechanical methods for the recovery of metal values from aluminum skim one of the best is the centrifuge method cited above. The centrifuge method of U.S. Pat. No. 4,137,073 produces an excellent recovery of metal and also produces a low density residue which is much easier to process mechanically to achieve additional useful recoveries than is the residue from pressing or rolling processes which, of necessity, compact their residues into dense solids.
The centrifuge method was tested and confirmed in its recovery and residue processing at prototype scale (at up to about 450-pounds [about 200-kg] of skim per batch) in several commercial plants. To date the centrifuge method has not been used commercially because of problems with the scale up of the equipment to handle unbalanced loads of hot aluminous skim of a practical commercial size i.e. weighing about 1,000-pounds (about 500-kg) or more.
A scale up of apparatus, made as shown in FIG. 6 of the aforesaid patent, was attempted and was abandoned. Some of the unsolved problems encountered in the scale-up were reputed to be difficulty in controlled spinning of an initially unbalanced load (the need for speedy loading of the bowl with skim was to conserve heat and to conserve labor time; speedy loading tended to preclude balancing the load), difficulty of maintaining a controlled spin of the bowl with the changing mass as major amounts of the enriched liquid phase left the rotating bowl and the enriched in solids phase moved and shifted with the bowl during spinning. Hence, this superior centrifuge method has not been used for the reclamation of aluminum skim because the apparatus to practice the method as illustrated in said method patent was not deemed commercially practical by production plant management after it was tried.
The prior art describes many means to centrifuge metal or metal containing mixtures. One company, the Danish Bergsoe Group, has made centrifuges for primary and secondary smelters of non-ferrous metals; however their equipment is covered by a secrecy clause and "must not be sold to any other company without their permission. "Neither must it be copied, and spares--if any--should be ordered from our company"; hence, their apparatus is not known to the art. Others, particularly those in the galvanizing industry, have used centrifuges to remove excess zinc from coated parts e.g. U.S. Pat. Nos. 1,779,366; 2,428,840; and 3,699,918 and others to treat articles in a bath such as U.S. Pat. No. 3,814,117. Galvanizing centrifuging operations are distinctly different from those of aluminum skim reclamation for several fundamental reasons: the amount of liquid to be removed in a galvanizing operation is only that small amount of excess liquid that does not readily drain from the zinc coated, steel parts, thus the amount of liquid removed does not involve more than about 10% by weight of the centrifuge charge, further, the main mass of the centrifuge charge is solid steel parts which are preloaded and usually do not suffer appreciable shifting about during centrifuging; hence, the problems and solutions of the galvanizing art would not be expected to be even remotely suggestive of solutions for centrifuging a semi-solid, usually initially unbalanced, load of mushy, aluminum skim preferably containing 50% or more of separable liquid which results in a bowl-retained, solids-enriched residue which is typically found pressed against the outer walls of the bowl with a center cylindrical region empty.
The prior art also describes many means to drive centrifuges, for example: from below, from the side or from above as in U.S. Pat. No. 733,738. Nevertheless there has not been a single, successful, commercial operation of a skim centrifuge utilizing my invention of the method of U.S. Pat. No. 4,137,073 of which I am aware.
Accordingly, for over ten years what the art has sought, and by various means has attempted to find, is a simple, efficient, rugged apparatus with which to practice the centrifuge method of U.S. Pat. No. 4,137,073 for the recovery of liquid aluminum from hot, aluminum skim. It is an object of this invention to describe a novel apparatus which provides the means to successfully, simply, ruggedly, and efficiently practice the said skim reclamation method.
It is a further object of this invention to describe a novel apparatus which can be used for the centrifugation of other large, initially unbalanced masses of mixed solids and liquids to permit the separation of the mixture into beneficiated portions which involves the removal from the mixture during rotation of at least 20% and up to about 90% of the mixture as a liquid phase and in which a redistribution of the mass of the solid-rich, retained-phase can be accomplished smoothly and without undesirable perturbations of the centrifuge during operation.